A big contributing factor to this (although there almost certainly others) is Quebec, and more specifically, the quiet revolution.

For some brief historical background, the French Canadians living in Canada were not assimilated as they were in the USA, allowing them to retain a distinct religion, language, and culture compared to what would later become other Canadian provinces. Despite British efforts following the seven years war, Quebec was always a majority french speaking Catholic province.In addition, French Canadians composed a much higher relative percentage in Canada than in the USA throughout the 19th century, which will be important for what I'm about to discuss. Even today, roughly 25% of Canada's population is french speaking as the first official language, a vast majority of whom have at least some French Canadian heritage.

Fast forward to 1960. The longtime Quebec Premier, Maurice Duplessis, is dead, renowned for his policies of working with the church, who at this point had very large influences in Quebec's social infrastructure, including hospitals and schools. As well in Quebec, there was a longstanding trope (with a certain degree of merit, although I should note that the majority of English people were not necessarily wealthy, not business owning) that the "english" had the money while the French were subservient, with French employees often being told to "speak white" by their White Anglo Saxon Protestant patrons, all the while the Catholic church utterly dominated all aspects of daily life.

Jean Lesage is elected premier, on the plateforme "maitres chez nous" translating roughly to "masters of our homes". Over the course of the next ~40 years (and still Continuing to today), but especially throughout the 1960s, Quebec society undertook a new direction. This included getting church out of social life via nationalisation of social institutions (the creation of the ministry of education for example), transforming hospitals into secular Institutions, the Nationalisation of hydroelectricity and the promotion of French Canadian owned business. The 1940s and 1950s, during which the Duplessis government ruled for the better part of 2 decades, was soon referred to as "le grand noirceur" or "the the great blackness".

Throughout all of this change, many French speaking Quebecers (quite notably the baby boomer generation and gen x) began to see the church to blame, with Gen X being educated in institutions that were no longer church controlled. The church was rapidly seen as an agent of oppression along with the English bourgeoisie, seen as keeping French Canadians religious and uneducated. The term used by Québec is "laïc" which goes a step farther than secularism, but rather actively discourages any and all religion in government function. One aspect of that in modern times is swearing in Quebec. The Canadian brand of french has employed the church to use as swear words, in which the French translations of chalice, the Tabernacle, and the host are common curses. This in turn was transferred to millenials. In the 1990s, Denomination schoolboards were scrapped altogether, replacing Catholic and Protestant schoolboards with French and English ones. In order to respect the 20 year rule of the subreddit, I won't speak about religious beliefs today, but the trends of rejecting the church has certainly persisted to younger generations.

This is a reminder to anyone interested in answering the question that all answers must focus on events and data from more than 20 years ago in accordance with our rules.

There are many factors that might have contributed to this. The decline in religiosity happened earliest for mainline protestants; Canada had a larger proportion of mainline protestants, since the evangelical movement never really took off there. Canada was also more urbanized for much of the late 20th century, which tends to reinforce declines in religiosity. Mormons, Latinos, and african-americans are all more religious than average, and are all much more common in America than Canada, which might also play a part.